package com.wc.AlgoOJ_train.train1.c5.A501;

import java.util.Scanner;

/**
 * @Author congge
 * @Date 2024/1/10 19:41
 * @description 改革春风吹满地
 * http://43.138.190.70:8888/training/6554d232dc9f4e711d9836a0
 */
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNextInt()) {
            int n = sc.nextInt();
            if (n == 0) {
                return;
            }
            int[] x = new int[n + 1];
            int[] y = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                x[i] = sc.nextInt();
                y[i] = sc.nextInt();
            }
            long a = 0;
            long b = 0;
            for (int i = 1; i < n; i++) {
                a += (long) x[i] * y[i + 1];
                b += (long) y[i] * x[i + 1];
            }
            a += (long) x[n] * y[1];
            b += (long) y[n] * x[1];
            double ans = 1.0 / 2 * Math.abs(a - b);
            System.out.printf("%.1f\n", ans);
        }
    }
}
